Hans-Reimer Rodewald at the University of Ulm, Germany, and his colleagues
say they have discovered a new organ - a thymus the size of a pin-head - in
the necks of mice. Previously, the mouse was thought to have just one
thymus, situated near the heart.

The thymus helps to supply the immune system with T cells, and this
unexpected finding raises questions about studies that have used mice with
the main thymus removed to investigate, for example, T-cell production in
other organs, such as the gut and skin.